This invention relates in general to sealing systems for liquid storage tanks and, in particular, to a sealing system for preserving the integrity of liquid contained in a liquid storage tank having a roof freely floating on the liquid contained within the storage tank.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a combined weather shield and vapor seal for sealing the opening or space between the floating roof and the inner wall of a storage tank.
When storing liquids in large containers, such as storage tanks of the type having a roof which floats on the liquid contained in the storage tank, it is necessary to prevent the liquid from becoming contaminated by rain, snow, ice, dust, dirt and the like, and to prevent loss of the liquid stored therein through evaporation or vaporization. Evaporation losses are not only costly due to the loss of the stored liquid, but may be dangerous or contaminate the environment. To this end vapor loss is closely controlled by government pollution control agency regulations which require the sealing system to meet specific emission control or vapor loss standards. The sealing system of the present invention has met the most stringent requirements for emission control and vapor loss standards, including Rule 463 of the California Air Resources Board.
Since the level of liquid contained within such storage tanks frequently changes, the roof is designed to rise and fall with the liquid contained in the storage tank. In order to allow the storage tank roof to freely rise and fall with the level of the liquid contained in the tank, the roof is designed with a diameter less than the inner diameter of the tank shell. Contaminants are prevented from entering the liquid by a weather shield which covers the rim space extending between the roof or pontoon and the inner wall of the tank or shell. A vapor barrier or seal is used to prevent evaporation of the liquid through this same rim space.
In order to meet these requirements, the storage tank sealing system must conform to any distortions or irregularities in the inner walls of the storage tank shell. In addition, since the tank roof or pontoon floats on the liquid contained within the storage tank, the roof will shift due to wind loading which destroys concentricity between the roof and tank shell. Therefore, it is known to those skilled in the art that the sealing system must allow for such movement between the roof and the inner wall of the shell in order to maintain an effective seal throughout the extremes of such wind loading induced movement as well as to compensate for distortion and irregularities in the tank shell.
Many and various approaches have been taken in an attempt to provide a satisfactory sealing system for storage tanks having a floating tank roof. For example, the sealing systems disclosed in U.S. Pat. No. 3,589,549 and U.S. Pat. No. 3,119,511 utilize separate structure to function as a primary or vapor seal, and a secondary or weather shield. With seals of this type the primary or vapor seal comprises an annular tubular casing, typically of self-supporting resilient material of a cellular structure, which is pressed into sealing engagement with the inner wall of the tank shell to form a vapor tight seal to prevent evaporation losses from the tank. A weather shield, which has a flexible or resilient bumper carried on the outermost edge of an annular array of individual plate-like hood segments, is secured to the upper portion of the floating roof and extends a length sufficient to maintain engagement with the inner wall of the tank shell to be effective throughout the conditions previously described. Other examples of the use of separate primary and secondary seals are disclosed in U.S. Pat. No. 1,992,221; U.S. Pat. No. 3,373,891; and U.S. Pat. No. 3,900,127. In each of these prior art systems the primary seal which forms a vapor barrier comprises a flexible, vapor-impermeable fabric extending between the floating roof and the inner wall of the cylindrical tank shell to prevent loss of the liquid by evaporation.
A further attempt to provide a successful sealing system is disclosed in U.S. Pat. No. 1,879,572 wherein a plurality of spaced adjacent spring steel sections are inclined outwardly from the upper portion of the floating roof a distance substantially further than the space between the roof and the tank wall. The adjacent sections are spaced laterally to provide for horizontal expansion and contraction of the tank and distortion of the sections. A flexible, pliable web, preferably of fabric, is secured to each of the laterally spaced sections.
With these prior art sealing systems, utilizing separate primary and secondary seals, in order to inspect the primary or vapor seal to insure that it is still effective and does not require repair, the secondary or weather shield seal must be moved to give access to the vapor seal. In such sealing systems it is necessary that the weather shield be readily positionable to allow inspection of the vapor seal without requiring extensive labor, or causing damage to the weather shield, when it is necessary to inspect or repair the vapor seal.
Another problem associated with many prior art sealing systems which utilize a separate primary and a separate secondary seal is that the secondary seal is sometimes attached to the primary seal. Such attachment does not allow for easy inspection. Furthermore, in the event the vapor seal becomes distorted or loses its sealing capabilities, the weather shield is not sufficient to provide a vapor barrier. Such systems frequently require that the seals be removed from the tank if the tank shell structure itself ever needs repair, because burning or welding on the tank structure will destroy the seals.
Certain attempts, such as disclosed in U.S. Pat. No. 3,373,831 and U.S. Pat. No. 4,116,358, have been made to utilize a single sealing structure as both a vapor barrier and a weather shield. In U.S. Pat. No. 3,372,831 a resilient material forms a wiper seal and is held in contact with the inner wall of the tank by an intermittently spaced series of separate arcuate strips which reinforce the wiper seal. While such a wiper seal (referred to in the art as a passive seal), may be somewhat effective, typically vapor pressure builds up under the seal until the vapor blows by the seal and escapes into the atmosphere. In addition, such a seal does not provide a sufficient centering force for resisting wind loading imposed on the tank roof.
The sealing system disclosed in U.S. Pat. No. 4,116,358 utilizes a single structure as both a weather shield and a vapor seal. A plurality of overlapping or abutting shield sections, preferably made of metal, are secured at one end to the upper portion of a floating roof and extend outwardly a length sufficient to form a sealing engagement with the inner wall of a tank in the manner disclosed in U.S. Pat. No. 1,879,572 and U.S. Pat. No. 1,992,221. A flexible wiper blade, preferably of rubber, is secured to the free end of the shields, in a manner acknowledged in the patent to be known in the prior art, to engage the inner wall of the storage tank shell. An annulus of a gas-impermeable fabric covers the shield, about the perimeter of the roof, and overlies the shield sections to cover the interfaces between adjacent sections of the shield to prevent the loss of vapors from the interior of the tank to the atmosphere.
While such sealing systems have succeeded in overcoming certain problems encountered with separate primary and secondary sealing structures, the abutting or overlapping of shield sections requires an annulus of vapor-impermeable fabric to overlie the shield, extending between sealing engagement with the top of the floating roof and the flexible wiper which overlies the shield, in order to prevent vapor loss through the interface between adjacent shield sections. Since this vapor barrier forming fabric is carried on the outside of the shields, the vapor barrier deteriorates fairly quickly from such things as ozone, ultraviolet light, and atmospheric contaminants which are always present in the environment where such tanks are usually built. As a result, it has been found that when the fabric deteriorates the vapor which is intended to be trapped beneath the fabric permeates the fabric and is discharged into the atmosphere.